1. Field of the Invention
The present invention relates to a piezoelectric type inkjet head chip for deforming a piezoelectric element through voltage application and boosting a channel internal pressure to discharge an ink droplet through a nozzle hole, a driving method for an inkjet head chip, an inkjet head, and an inkjet recording apparatus.
2. Description of the Related Art
As to the above-mentioned piezoelectric type inkjet head chip, there has been conventionally known a structure including an ink chamber for containing ink, a plurality of piezoelectric elements deformable through voltage application, a plurality of channels partitioned with the piezoelectric elements and formed parallel to each other, and nozzle holes communicating with the channels for discharging an ink droplet toward a recording medium.
Specifically, the inkjet head chip includes an ink chamber plate including the ink chamber formed on one surface thereof, an actuator plate including the plurality of channels formed on one surface thereof, and a nozzle plate including a plurality of the nozzle holes formed in a row, in which another surface of the ink chamber plate and the one surface of the actuator plate are bonded to each other so that the ink chamber plate overlaps the actuator plate, and the nozzle plate is bonded to one end of the actuator plate in a channel longitudinal direction thereof. Ink introduction holes are formed in the ink chamber plate, and the ink chamber and the channels are communicated with each other through the ink introduction holes, whereby the ink contained in the ink chamber is supplied to the channels. According to the inkjet head chip with the structure as described above, a voltage is applied to the piezoelectric element for deformation, and a volume of the channel partitioned with the deformed piezoelectric element is contracted to boost a channel internal pressure, to thereby discharge the ink contained in the channel. Accordingly, the ink droplet can be sprayed onto the recording medium.
The above-mentioned inkjet head chip is classified into a shared wall type inkjet head chip in which discharge channels (channels communicating with the nozzle holes) are successively disposed side by side and an independent channel type inkjet head chip in which discharge nozzles and non-discharge nozzles (channels not communicating with the nozzle holes) are alternately disposed side by side.
In the above-mentioned inkjet head chip, it is desired that discharge speeds of the respective nozzle holes disposed in a row be made uniform for improving image quality of a printed matter by an inkjet printer.
For this purpose, there has been conventionally proposed a technology of optimizing a driving waveform as described in JP 2006-224545 A. This technology is applicable to the shared wall type inkjet head chip. In this technology, when a discharge channel which discharges ink sporadically or intermittently does not discharge ink, a pulse which is so short that ink is not discharged is applied in response to a restoration timing of the discharge channel which has discharged ink. This technology solves a problem that an ink discharge speed of a nozzle hole of a discharge channel which discharges ink sporadically or intermittently is slower than an ink discharge speed of a nozzle hole of a discharge channel which discharges ink successively. Accordingly, ink discharge speeds of the respective nozzle holes can be made uniform.
However, in the above-mentioned conventional technology, a difference in ink discharge speed of the nozzle hole is generated between discharge channels disposed in a middle portion of the discharge channels in a channel parallel direction and discharge channels disposed in both end portions thereof in the channel parallel direction, which causes a problem that ink discharge speeds of the respective nozzle holes cannot be made sufficiently uniform. More specifically, in the shared wall type inkjet head chip, discharge speeds of the nozzle holes disposed in both end portions of the inkjet head chip are slower compared with the nozzle holes disposed in a middle portion thereof, and in the independent channel type inkjet head chip, discharge speeds of the nozzle holes disposed in both end portions of the inkjet head chip are faster compared with the nozzle holes disposed in a middle portion thereof. As to the cause of the difference in ink discharge speed as described above, it is conceivable that an electrical condition is different between the discharge channels disposed in the both end portions and the discharge channels disposed in the middle portion. In other words, a number of other discharge channels are formed on both sides of each of the discharge channels disposed in the middle portion, and thus an electric field applied to the other discharge channels affects the discharge channels disposed in the middle portion from the both sides thereof. On the other hand, a number of other discharge channels are formed only on one side of each thereof, and hence an electric field applied to the other discharge channels affects the discharge channels disposed in the both end portions only from the one side thereof.